1. Field of the Invention
The present invention generally relates to an apparatus for controlling a variation in torque of an internal combustion engine, and more particularly to a torque variation control apparatus which controls a predetermined parameter of the internal combustion engine so that the amount of intercycle variation in torque of the internal combustion engine is maintained within an allowable torque variation amount range.
2. Description of the Related Art
As is well known, various apparatuses have been proposed which intend to improve the fuel economy of an internal combustion engine and reduce the amount of nitrogen oxides (NOx) therein. Japanese Laid-Open Patent Publication No. 2-67446, for example, discloses an apparatus which measures the amount of intercycle variation in torque of the internal combustion engine and controls a predetermined engine control parameter so that the measured intercycle torque variation amount becomes equal to a target torque variation amount. Some features of conventional methods are, for example, that the air-fuel ratio is controlled so that a mixture of air and fuel is as lean as possible, or that an exhaust gas recirculation system is controlled so that an increased amount of exhaust gas s fed back to an intake manifold.
More specifically, the apparatus disclosed in the above Japanese publication detects only a decrease in the torque for each cycle and accumulates decreases in the torque for a predetermined number of cycles. An accumulated amount is defined as the amount of torque variation (a torque variation amount). The torque variation amount is compared with a target torque variation amount (torque variation decision value), and a predetermined engine control parameter, such as the air-fuel ratio or the amount of recirculated exhaust gas, is controlled on the basis of the result of comparison.
In the above-mentioned torque variation control apparatus, there is a response delay until a controlled amount of fuel is actually injected into an intake system. There is also a response delay until a controlled amount of exhaust gas is actually supplied to the intake system. In order to prevent hunting arising from the above response delay, an allowable torque variation amount range including a target torque variation amount is defined. In actuality, the allowable torque variation range is determined, taking into account a dispersion in the torque variation amount.
FIG. 1 is a graph of a torque variation amount vs. air-fuel ratio (or the amount of recirculated exhaust gas) characteristic curve I. The torque variation amount in FIG. 1 is measured by means of a combustion pressure sensor. A line indicated by II is the target torque variation amount (torque variation decision value). The characteristic curve I has a sharp slope when the torque variation amount is greater than the torque variation decision value II because the combustion reaction is unstable. When the torque variation amount is small, the characteristic curve I has a gentle slope because the combustion reaction is stable. Hence, when the torque variation amount is large, it is easy to determine whether or not the torque variation amount is greater than the torque variation decision value II. However when the torque variation amount is smaller than the torque variation decision value II, particularly when the torque variation amount is close to a lower limit of an allowable torque variation range (dead range) III, it is very difficult to determine whether or not the torque variation amount is within the allowable torque variation range III because the characteristic curve I has a gentle slope.
When the detected torque variation amount is within the allowable torque variation range III, the control (combustion reaction) is in the stable state. If the detected torque variation amount corresponds to a point A in the stable state, the air-fuel ratio (or the amount of recirculated exhaust gas) is maintained stably at a level "a" because A is within the allowable torque variation range III. However, it is desired that originally the air-fuel ratio be controlled to a lean level "b" (or that the amount of recirculated exhaust gas be controlled to a rich level of exhaust gas "b"). Hence, an amount of fuel corresponding to the difference between "b" and "a" is wasted, and emissions degrade by an amount corresponding to the difference between "b" and "a".